The Immigration and Checkpoints Authority (ICA) said in a Facebook post it was "experiencing slowness at selected passenger clearance checkpoints", leading to the delays.

It advised travellers to postpone all non-essential trips outside the country.

Long lines formed at Changi, often voted the world's best airport where clearing immigration is normally a breeze.

"It's very chaotic at T4 and very little communication," wrote one person on Facebook. His comment was accompanied by a photo showing passengers packing the terminal lobby.

Passengers whose flights were departing soon were being called to be cleared at the manual counters, one more traveller wrote.

There were also "super long" lines at the airport's Terminal 1 but manual counters were clearing them up fast, another traveller said.

"An immigration system disruption has affected the auto-lanes for departing and arriving passengers across the terminals," Changi Airport said in a statement.

It advised passengers to come to the airport earlier.

Long lines of vehicles also formed along two border crossings between Singapore and neighbouring Malaysia, among the busiest crossings in the world.

Selected automated lanes at those crossings were affected, ICA said.

However, the lines of passengers and vehicles appear to have eased by Friday early afternoon, according to AFP journalists.

ICA said in an update it recalled off-duty personnel to "provide additional support and to manage the situation to ensure law and order", and advised travellers "to cooperate with officers on-site."

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